Dental film packets are widely used for X-raying teeth. The usual form of such a packet is a sealed (i.e. waterproof) packet containing layers of X-ray sensitive film, metal foil, and black paper. The packet is held in place in the patient's mouth while the X-ray is being taken, and is then removed and the film is taken out and developed. The film obviously then shows the desired X-ray; two films are sometimes used, automatically giving two copies of the X-ray picture. The metal foil serves to absorb the X-rays after they have passed through the film, so reducing the total X-ray dosage to the patient. The black paper eases the handling of the film when it is being taken out of the packet tg be developed.
Various forms of envelopes are known for providing the outer sealing of the packet. The usual current form is made of plastics material, typically polyvinyl chloride, and is produced by stamping. Two webs of the plastics material are interleaved with film packs and are fed through a machine in which the two webs are heat-sealed together around each film pack and the resulting packet is simultaneously separated from the webs and the other packets being formed. The manufacture of packets using such envelopes is readily automated and the resulting packets are inexpensive.
As stated above, the packets are intended for intra-oral use. The packet may be supported in a clip on the end of a rod, and held in position in the patient's mouth by the patient gripping the rod between his teeth, or it may simply be held in position by the patient's finger. The X-ray beam is directed from outside the mouth through the teeth and onto the packet.
Packets made in the manner outlined above tend to have somewhat hard and sharp edges due to the heat-sealing of the two layers of plastics material. This is a drawback, because such edges can cause discomfort to the patient, particularly if the patient has a sensitive lining to the mouth (as may easily be the case for someone requiring dental treatment).
Several proposals have involved providing a band of some sort around the periphery of the envelope, to act as a cushion around the envelope's hard and sharp edge; among such proposals are WO 90/02358 (Liese/Eastman Kodak) and U.S. Pat. No. 4,852,143 (Scheier at al), both of which contain references to other somewhat similar proposals. A packet having such a band will therefore not scratch the patient.
However, such bands have their own disadvantages.
Sometimes the band absorbs moisture and may therefore dry the patient's mouth and cause discomfort in that way, and if the band expands on being moistened, its expansion may easily cause displacement of the packet from its desired position and may even cause discomfort if the expansion is too great.
The automatic manufacture of such a packet is not easy, while manual manufacture is inconvenient, requires particular care to avoid possible contamination, and is expensive. If the band is not made integrally with the envelope, there will be a space along the inside of the band, between it and the envelope, which can be unhygienic, while making the band integrally with the envelope presents manufacturing difficulties and expense. The band can easily trap saliva and so make the exposed packet unpleasant to handle after use.
It has also been proposed, in U.S. Pat. No. 4,922,511 (Gay/Eastman Kodak) , to provide a heat-sealed packet in which the envelope has an exceptionally wide margin around the central part containing the films. The idea is that this margin of the envelope will bend back on itself in use, so that the tissue of the patient's mouth will be contacted by the curved outer parts of the bends of the margins rather than by the hard edges of the packet.
This, however, increases the size of the packet, and this and the loss of rigidity makes it less convenient to handle, and in particular, it adds significantly to the size of a package, giving rise to inconveniences in bulk storage and supply of such film packets. Also, it may present inconveniences on being inserted into the patient's mouth. If the outer parts of the margin of the two layers of the plastics material used for the envelope are sealed together, then the rigidity of the margin will be substantial, and its width must be particularly great to achieve adequate flexibility, while if the sealing is confined to the inner parts of the margin, the two layers will form a loose crack at the outer parts of the margin which will tend to pick up moisture and so be unpleasant to handle and potentially unhygienic. Also, the folding or bending back of the margin is unlikely to be uniform around the whole of the edge of the packet, thereby producing kinks, and the larger size of the packet compared to a normal packet means that any potential for kinking is likely to be more serious.